This invention relates in general to electric motors and in particular to an improved stator pole structure for a variable reluctance electric motor.
Electric motors are well known devices which convert electrical energy to rotary mechanical energy. To accomplish this, electric motors establish and control electromagnetic fields so as to cause the desired rotary mechanical motion. There are many different types of electric motors, each utilizing different means for establishing and controlling these electromagnetic fields. Consequently, the operating characteristics of electric motors vary from type to type, and certain types of electric motors are better suited for performing certain tasks than others.
Synchronous motors constitute one principal class of electric motors. The two basic components of a synchronous motor are (1) a stationary member which generates a rotating electromagnetic field, generally referred to as the stator, and (2) a rotatable member driven by the rotating magnetic field, generally referred to as the rotor. Synchronous motors are characterized in that the rotational speed of the rotor is directly related to the frequency of the electrical input signal applied thereto and, therefore, the rotational speed of the electromagnetic field generated thereby. Thus, so long as the frequency of the applied electrical input signal is constant, the rotor will be driven at a constant rotational speed. Within this broad definition, however, the structure and operation of synchronous electric motors vary widely.
One variety of synchronous electric motor is a variable reluctance motor. Variable reluctance motors operate on the principle that a magnetic field which is created about a component formed from a magnetically permeable material will exert a mechanical force on that component. This mechanical force will urge the component to become aligned with the magnetic flux (lines of force) generated by the magnetic field. Thus, by using the stator to establish and rotate a magnetic field about a rotor formed from a magnetically permeable material, the rotor can be driven to rotate relative to the stator. The resistance to the passage of this magnetic flux from the stator to the rotor is referred to as reluctance. The magnitude of this reluctance changes with the rotational position of the rotor relative to the stator. Thus, electric motors of this type are commonly referred to as variable reluctance motors.
In a conventional variable reluctance motor, the stator is generally hollow and cylindrical in shape, having a plurality of radially inwardly extending poles which extend longitudinally throughout the length thereof. Similarly, the rotor is generally cylindrical in shape, having a plurality of radially outwardly extending poles which extend longitudinally throughout the length thereof. The stator and the rotor are both formed from a magnetically permeable material. A winding of an electrical conductor is provided about each of the stator poles. By passing pulses of electrical current through each of the stator windings in a sequential manner, the stator poles can be selectively magnetized so as to attract the rotor poles thereto. Consequently, the rotor will rotate relative to the stator.
Because of the structural geometry of conventional variable reluctance motors, it is desirable that the magnetic flux generated by the electromagnetic fields extend circumferentially throughout the stator and the rotor in a plane which is generally perpendicular to the longitudinal axis of the motor. Any magnetic flux which extend parallel to the longitudinal axis of the motor (often referred to as eddy currents) do not contribute to the attraction of the rotor poles toward the stator poles and, therefore, reduce the overall performance of the motor. To reduce the magnitude of these undesirable eddy currents, it is known to form both the stator and the rotor from a plurality of relatively thin laminations of a magnetically permeable material. Each of these laminations has a cross sectional shape which corresponds to the desired cross sectional shape of the stator and the rotor. The laminations are secured together to form the stator and the rotor.
When selecting any kind of electric motor for use in a particular application, several basic considerations are important. One such consideration is the efficiency of the motor, i.e., the ratio of the mechanical output power (torque in rotary electric motors and force in linear electric motors) to the electrical input power. A second consideration is the maximum amount of torque or force which can be generated by the electric motor. A third consideration is the physical size of the electric motor. Obviously, it would be desirable to increase the efficiency and output torque of the electric motor, while reducing (or at least not increasing) the physical size thereof. A fourth consideration is the amount of variation in the torque generated by the motor during operation, often referred to as torque ripple. Torque ripple occurs as a result of the sequential energization and de-energization of the various phase windings of the motor. Ideally, the amount of this torque ripple is maintained at a minimum during operation such that the torque generated by the motor is essentially uniform.